1. Field of the Invention
U.S. Pat. No. 6,146,794 issued Nov. 14, 2000, the entire disclosure of which is incorporated herein by reference.
2. Description of the Related Art
In an electron beam exposure apparatus, when electrons are charged at a photomask, the focus of electron beams is bent by the charged electrons, so that it is impossible to depict an accurate pattern on the photomask.
On the other hand, a photomask is generally constructed of glass substrate, an optical shield layer made of Cr on the glass substrate, a reflection avoiding layer made of CrO on the optical shield layer, and an electron beam resist layer on the reflection avoiding layer. This photomask is set in a cassette of an electron beam exposure apparatus for depicting a pattern thereon.
In order to prevent electrons from being charged at the photomask, i.e., the optical shield layer thereof, conductive pins supported by springs at the cassette are inserted into the photomask. Thus, electrons charged at the optical shield layer are effectively discharged from the conductive layer. In this case, if a force applied to the conductive pins is insufficient, it is impossible for the conductive pins to penetrate the electron beam resist layer and the reflection avoiding layer. As a result. the conductive pins cannot be in contact with the optical shield layer. Contrary to this, if the force applied to the conductive pins it too strong, an excessive force is applied to the photomask. As a result, the photomask is distorted or deformed, so that it is also impossible to depict an accurate pattern on the photomask.
In a first prior art antistatic method for a photomask, a reflection avoiding layer is reduced in size. as compared with an optical shield layer. As a result, conductive pins easily penetrate the electron beam resist layer to reach the optical shield layers since there is no reflection avoiding layer beneath the conductive pins (see JP-A-4-371952). This will be explained later in detail.
In the first prior art photomask, however, since the formation of the reflection avoiding layer on the optical shield layer by an evaporation process needs a holder for covering the periphery of the optical shield layer, the thickness of the reflection avoiding layer becomes non-uniform particularly around the edge of such a holder.
In a second prior art photomask, a conductive pin is in contact with an electron beam resist, and in addition, a high power supply electrode penetrates the electron beam resist and reaches an optical shield layer. In this state, when a high power supply voltage is applied to the high power supply electrode, the portion of the electron beam resist layer between the conductive pin and the optical shield layer is electrostatically destroyed, so that the conductive pin is electrically in contact with the optical shield layer (see JP-A-2-125416). This will be also explained later in detail. The second prior art photomask, however, makes the apparatus complex, thus increasing the manufacturing cost.
It is an object of the present invention to provide an improved antistatic method for a photomask without reducing the size of a reflection avoiding layer and without a complex apparatus.
According to the present invention, in an antistatic method for a photomask including a conductive optical shield layer, at least two conductive pins are inserted into the photomask, so that the conductive pins are in contact with the conductive optical shield layer. Then, the photomask is set in a cassette of an electron beam exposure apparatus. Then, the conductive pins are electrically connected to the cassette by conductive plates. Thus, electrons charged at the conductive optical shield layer by electron beams are effectively discharged from the conductive pins to the cassette.